The Consumer Price Index isn’t one price, it’s roughly 80,000 of them, sorted into a structure that decides how much any single price can move the whole number. A subway fare and a surgeon’s bill both count, but not equally: the CPI weights every item by its share of what American households actually spend, so a 10% jump in rent moves the index far more than a 10% jump in postage stamps. That weighting scheme, the “market basket,” is what how inflation is calculated glosses over in one paragraph and this article covers in full.
The eight major groups
The Bureau of Labor Statistics organizes every priced item under eight top-level groups. Their relative importance, the share of total spending each commands in the index, shifts a little every year but the rank order rarely changes:
| Major group | Approx. weight | What it covers |
|---|---|---|
| Housing | ~36% | Rent, owners’ equivalent rent, utilities, furniture, appliances |
| Transportation | ~16% | New and used vehicles, gasoline, auto insurance, airfares |
| Food and beverages | ~13% | Groceries, dining out, alcohol |
| Medical care | ~8% | Health insurance, doctor visits, prescription drugs, hospital services |
| Recreation | ~5% | Streaming services, sporting goods, pets, admissions |
| Education and communication | ~6% | Tuition, childcare, phone and internet service |
| Apparel | ~2% | Clothing and footwear |
| Other goods and services | ~3% | Personal care, tobacco, life insurance, funeral expenses |
Energy (gasoline, electricity, natural gas) and food aren’t separate major groups; they’re carved out of housing, transportation, and food and beverages specifically so the BLS can publish core inflation, the index minus those two volatile slices.
Housing dominates for one reason: OER
Housing’s ~36% weight is more than double the next-largest group, and most of that is a single line item: owners’ equivalent rent (OER), the estimated amount a owner-occupied home would rent for. Roughly one in four dollars in the entire CPI is OER, not because homeowners write a literal rent check but because the BLS treats housing services, not the home’s sale price, as the thing being consumed. A home’s purchase price behaves like a stock trade, an investment, not consumption, so it doesn’t enter the index at all. This is the single most misunderstood fact about the CPI: home prices can double and the index won’t directly register it, only the rental value does.
Where the weights come from
Nobody guesses these percentages. Every one to two years, the BLS’s Consumer Expenditure Surveys ask tens of thousands of households to log what they bought, down to the item. Those diaries and interviews get aggregated into the relative importance table above. Since 2023 the weights update annually instead of every two years, specifically so the basket keeps pace with fast-moving habits (more streaming subscriptions, less cable, more ride-hailing, less traditional taxi spending) without a multi-year lag.
Inside a category: how granular it gets
“Transportation” isn’t one price any more than “housing” is. The BLS breaks each major group into expenditure categories, then item strata, then individual entry-level items, the actual thing a price collector prices in a store. A few examples of that drill-down:
- Transportation splits into new vehicles, used vehicles, motor fuel, motor vehicle insurance, and public transportation, each tracked and weighted separately before rolling up.
- Food and beverages splits into food at home (itself broken into cereals, meats, dairy, fruits and vegetables, and more) and food away from home (limited service meals vs. full service meals).
- Medical care splits into medical care commodities (drugs, medical equipment) and medical care services (physician services, hospital services, health insurance).
This granularity is why the monthly CPI report can tell you that egg prices moved differently than beef prices even though both sit inside “food at home”: each entry-level item carries its own weight and its own measured price change, and only the final roll-up produces the single headline number everyone quotes.
What’s excluded
The basket is deliberately a consumption basket, which rules out several things people assume are in it:
- Income and payroll taxes: not a purchase, so not priced.
- Investments: stocks, bonds, and real estate purchases are savings and asset allocation, not consumption. (Life insurance’s savings component is excluded too; only its service charge counts.)
- Home purchase prices: covered above, replaced by owners’ equivalent rent.
Where the prices come from
The basket’s weights answer how much each category matters; a separate BLS operation, monthly visits to thousands of retail outlets, service providers, and websites across about 75 urban areas, answers how much prices moved. The full mechanics of that collection process, plus how individual price changes get combined into the published index number, are covered in how inflation is calculated. The Fed, notably, doesn’t target this basket at all; it watches a differently weighted one, explained in CPI vs. PCE.
Why the weights matter for your own number
The CPI’s weights describe an average urban household, not your household. If you rent in a city with steep rent growth, housing’s roughly one-third weight probably understates your personal experience. If you own your car outright and rarely fly, the 16% transportation weight overstates yours. That gap between the published rate and any individual’s lived rate isn’t a flaw in the CPI, it’s the tradeoff of publishing one number for a country of over 330 million people; the methodology page documents exactly how this site turns that one number into every dollar comparison it publishes, including what a 1980 dollar is worth today.